Method for auto-ignition operation and computer readable storage device

ABSTRACT

The invention relates to an internal combustion engine comprising a fuel injector ( 2 ) for each cylinder; a fuel injection control unit ( 4 ) for controlling fuel injection quantity and a piston ( 5 ) in each cylinder whose compression action causes a mixture of air and fuel to be ignited. The engine is further provided with inlet and outlet valves ( 6, 7 ) and various sensors ( 12-16 ) for measuring various engine operating parameters. During compression ignition mode, the control unit ( 4 ) is arranged to select a λ-value from a map stored in the control unit, which value is a function of engine load and engine speed, and to compare the actual λ-value with the selected λ-value; whereby the control unit is arranged to adjust the intake manifold pressure as a function of the difference between the said λ-values in order to obtain the selected λ-value. The invention further relates to a method for operating the engine and a computer readable storage device ( 4 ).

The present application claims priority to European Patent ApplicationNo. 02029091.2, filed Dec. 30, 2002, titled INTERNAL COMBUSTION ENGINE,METHOD FOR AUTO-IGNITION OPERATION AND COMPUTER READABLE STORAGE DEVICE,naming Hans Ström and Lucien Koopmans as inventors, and claims priorityto European Patent Application No. 02029060.7, filed Dec. 30, 2002,titled INTERNAL COMBUSTION ENGINE, METHOD FOR AUTO-IGNITION OPERATIONAND COMPUTER READABLE STORAGE DEVICE, naming Hans Ström and LucienKoopmans as inventors, the entire contents of which are incorporatedherein by reference.

BACKGROUND AND TECHNICAL FIELD

The invention relates to an internal combustion engine that can beoperated in a homogeneous charge compression ignition combustion mode,as well as a method for controlling such an engine.

DETAILED DESCRIPTION

To improve thermal efficiency of gasoline internal combustion engines,lean burn is known to give enhanced thermal efficiency by reducingpumping losses and increasing ratio of specific heats. Generallyspeaking, lean burn is known to give low fuel consumption and low NO_(x)emissions. There is however a limit at which an engine can be operatedwith a lean air/fuel mixture because of misfire and combustioninstability as a result of a slow burn. Known methods to extend the leanlimit include improving ignitability of the mixture by enhancing thefuel preparation, for example using atomised fuel or vaporised fuel, andincreasing the flame speed by introducing charge motion and turbulencein the air/fuel mixture. Finally, combustion by auto-ignition, orhomogeneous charge compression ignition, has been proposed for operatingan engine with very lean or diluted air/fuel mixtures.

When certain conditions are met within a homogeneous charge of leanair/fuel mixture during low load operation, homogeneous chargecompression ignition can occur wherein bulk combustion takes placeinitiated simultaneously from many ignition sites within the charge,resulting in very stable power output, very clean combustion and highfuel conversion efficiency. NO_(x) emission produced in controlledhomogeneous charge compression ignition combustion is extremely low incomparison with spark ignition combustion based on propagating flamefront and heterogeneous charge compression ignition combustion based onan attached diffusion flame. In the latter two cases represented byspark ignition engine and diesel engine, respectively, the burnt gastemperature is highly heterogeneous within the charge with very highlocal temperature values creating high NO_(x) emission. By contrast, incontrolled homogeneous charge compression ignition combustion where thecombustion is uniformly distributed throughout the charge from manyignition sites, the burnt gas temperature is substantially homogeneouswith much lower local temperature values resulting in very low NO_(x)emission.

Engines operating under controlled homogeneous charge compressionignition combustion have already been successfully demonstrated intwo-stroke gasoline engines using a conventional compression ratio. Itis believed that the high proportion of burnt gases remaining from theprevious cycle, i.e., the residual content, within the two-stroke enginecombustion chamber is responsible for providing the hot chargetemperature and active fuel radicals necessary to promote homogeneouscharge compression ignition in a very lean air/fuel mixture. Infour-stroke engines, because the residual content is low, homogeneouscharge compression ignition is more difficult to achieve, but can beinduced by heating the intake air to a high temperature or bysignificantly increasing the compression ratio. This effect can also beachieved by retaining a part of the hot exhaust gas, or residuals, bycontrolling the timing of the intake and exhaust valves.

In all the above cases, the range of engine speeds and loads in whichcontrolled homogeneous charge compression ignition combustion can beachieved is relatively narrow. The fuel used also has a significanteffect on the operating range; for example, diesel and methanol fuelshave wider auto-ignition ranges than fuel. A further problem is toachieve ignition at a particular time with maintained combustionstability, while avoiding engine knocking, misfiring and increasedNO_(x) levels.

When the engine is operating in HCCI mode, a low fuel consumption has tobe sustained and or optimised. For low fuel consumption, the air/fuelratio has to be greater than stoichiometric, i.e. substantially 1 partfuel and 14 parts air. There is a trend for lower fuel consumption inthe direction of higher values of lambda if good combustion stabilitycan be sustained. A three way catalyst only converts unburnedhydrocarbons and carbon monoxide when the engine is operating lean(λ>1), hence NO_(x) emissions will be emitted to the atmosphere withoutafter-treatment.

High lambda values for optimised fuel consumption and NO_(x) emissionscan not be achieved with atmospheric pressure in the inlet manifold incombination with the inlet and exhaust valve settings used for HCCIcombustion.

Hence an object of the invention is to provide a means for controllingthe combustion process during auto-ignition, in order to maintain lowNO_(x) levels. Said means allows for monitoring of current combustionsand for correction of subsequent combustions dependent on the outcome ofthe monitoring process.

The above problems can be solved, in some cases, by an arrangement, amethod and a computer readable storage device for controllinghomogeneous charge compression ignition combustion, as described in moredetail below.

One embodiment relates to an internal combustion engine preferably, butnot necessarily, provided with variable valve timing (VVT), cam profileswitching (CPS), direct fuel injection (DI), and means for boosting themanifold absolute pressure (turbo, compressor etc.).

The following text will be mainly concentrated on embodiments includingthe above features. However, the general principle of the invention asclaimed is also applicable to, for instance, stationary aspiratingengines with fixed valve timing and a standard camshaft. Such enginesare often operated at fixed speeds and loads and are not subject to thetransients normally occurring in, for instance, engines for vehicles.

Also, although the following examples relate to fuels, an engineoperating according to principles of the invention can be adapted to usemost commonly available fuels, such as kerosene, natural gas, andothers.

The engine is possible to be operated on homogeneous charge compressionignition (HCCI) combustion mode. This is a combustion mode, differentthan conventional spark ignited (SI) combustion mode, in order to reducefuel consumption in combination with ultra low NO_(x) emissions. In thismode, a mixture containing fuel, air and combustion residuals iscompressed with a compression ratio between 10.5 and 12 to autoignition. The HCCI combustion has no moving flame front, incontradiction to a SI combustion that has a moving flame front. The lackof a flame front reduces temperature and increases the heat release ratehence increases the thermal efficiency of the combustion. The combustionresiduals are captured when operating the engine with a negative valveoverlap. Residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilute the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, cycle-to-cyclevariations (COV), noise and knocking combustion can be reduced. Thenegative valve overlap is achieved when the exhaust valve is closedbefore piston TDC and the inlet valve is opened after piston TDC in thegas exchange phase of the engine cycle.

The acquired valve timing for the negative overlap can be achieved byusing VVT and CPS, hence switching from conventional SI valve timing toHCCI valve timing with a shorter the valve opening duration and/or valvelift

In HCCI combustion mode one target is to keep NO_(x) emissions below thelegislative limit by using a control loop that controls the HCCIcombustion. NOx emissions, fuel consumption and combustion stability areaffected by the air fuel ratio of the mixture in the combustion chamber.A low air fuel ratio gives high fuel consumption and high NO_(x)emissions, while a high air fuel ratio gives low fuel consumption andlow HC emissions.

According to one embodiment of the invention, an internal combustionengine is provided with at least one cylinder and arranged to beswitched between spark ignition mode and compression ignition mode. Theengine comprises a fuel injector, through which fuel is injected into acombustion chamber, for each cylinder and a fuel injection control unitthat controls fuel injection quantity per combustion cycle injectedthrough each fuel injector. Fuel injection is preferably, but notnecessarily, achieved by means of direct injection (DI) into eachcombustion chamber. For the current invention, port injection is alsopossible.

A spark may be sustained in HCCI mode in order to keep the spark plugfrom fouling and, although the gas mixture is arranged to self ignite,contribute to an increased combustion stability and avoidance ofmisfire.

A reciprocating piston is arranged in each engine cylinder whosecompression action causes a mixture of air and fuel within thecombustion chamber to be ignited. Gas exchange is controlled by at leastone inlet valve preferably, but not necessarily, provided with variablevalve timing per cylinder for admitting a combustible gas, such as air,and at least one exhaust valve preferably, but not necessarily, providedwith variable valve timing per cylinder for exhausting combusted gases.

The combustion process is monitored by sensors for measuring air intakemanifold pressure and the air/fuel ratio, or λ-value, for the combustedexhaust gas, as well as standard sensors for engine load and enginespeed.

Other sensors may include temperature sensors for engine coolant and oilas well as NO_(x)-sensors and ion current sensors. If required, theengine may also be provided with engine knocking and combustionstability sensors. The knock sensor can be of the piezo-electric type,which may also be used for continuous monitoring of cylinder pressure.The combustion stability sensor may be an acceleration type sensor, suchas a flywheel sensor, or an ion current sensor. Alternatively, both saidsensors can be replaced by a single in-cylinder piezoelectric pressuresensor. By processing the output from such a sensor, it is possible toobtain a signal representing engine knock and a signal representingengine stability.

According to a further embodiment the engine is switched into acompression ignition mode. The control unit is arranged to select aλ-value from a map of λ-values stored in the control unit, which valueis a function of engine load and engine speed. The actual λ-value iscompared to the selected λ-value; whereby the control unit is arrangedto adjust the intake manifold pressure as a function of the differencebetween the said λ-values in order to obtain the selected λ-value.

High λ-values combined with low fuel consumption and low NO_(x)emissions can not be achieved with atmospheric pressure in the intakemanifold during HCCI operation at high loads and/or speeds. In oneembodiment the engine is provided with an intake air charging unit,arranged to adjust the intake manifold pressure.

The intake air charging unit may be a turbocharger provided with awastegate for controlling the intake manifold pressure. Alternativelythe intake air charging unit is a supercharger provided with a throttlefor controlling the intake manifold pressure.

According to a further embodiment, the intake air charging unit isarranged to increase the intake manifold pressure if the actual λ-valueis less than the selected λ-value. Similarly, the intake air chargingunit is arranged to decrease the intake manifold pressure if themeasured actual λ-value is greater than or equal to the selectedλ-value.

According to a further embodiment, the intake manifold is provided witha sensor for measuring intake air temperature. As the λ-value may varywith intake air temperature, the control unit is arranged to adjust theselected λ-value if necessary. The adjustment is based on the measuredintake air temperature compared to a reference temperature for thestored map.

According to a further embodiment, the invention relates to a computerreadable storage device having stored therein data representinginstructions executable by a computer to implement a compressionignition for An internal combustion engine, the engine having a pistondisposed in a cylinder to define a combustion chamber, intake valves foradmitting fresh air into the cylinder, a fuel injector for injectingfuel into the combustion chamber, exhaust valves for discharging exhaustgas resulting from combustion within the cylinder, wherein opening andclosing timings of the intake means and opening and closing timings ofthe exhaust means are adjustable, and sensors for measuring engine load,engine speed and an actual λ-value for the exhaust gas.

The computer readable storage device comprises:

-   -   instructions for adjusting opening and closing timings of the        intake means and opening and closing timings of the exhaust        means such that the piston reciprocates within the cylinder to        perform an exhaust phase, an exhaust gas retaining phase, an        intake phase, a compression phase, and an expansion phase;    -   instructions for providing a first start time of a first fuel        injection by the fuel injector during said exhaust gas retaining        phase and a second start time of a second fuel injection by the        fuel injector during said exhaust gas retaining phase;    -   instructions for selecting a λ-value from a map stored in the        control unit, which value is a function of engine load and        engine speed;    -   instructions for determining a difference between the actual        λ-value with the selected λ-value; and    -   instructions for adjusting the intake manifold pressure as a        function of the difference between the said λ-values in order to        obtain the selected λ-value

The computer readable storage device further comprises instructions fordetermining intake manifold pressure control signals indicative of theadjustment required by an intake air charging unit, as determined by thecontrol unit on the basis of comparison between the measured actualλ-value with the selected λ-value.

The storage device further comprises instructions for determining intakemanifold temperature by means of a temperature sensor and for adjustingthe λ-values in said stored map with respect to the measuredtemperature.

According to one embodiment, the engine is arranged to switch fromSI-mode to HCCI-mode when certain operating parameters are fulfilled.During compression ignition mode, the exhaust valve is arranged to closebefore top dead center during an exhaust stroke of the piston and theintake valve is opened after top dead center during a suction stroke ofthe piston. This creates a period of negative valve overlap, duringwhich exhaust and intake valves are closed. The fuel injection controlunit is arranged to control the fuel injection quantity so as to performa first fuel injection before top dead center of the piston stroke andto perform a second fuel injection after top dead center of the pistonstroke during the interval when both of the exhaust and intake valvesare closed. However, other examples of this so called split injectiontiming are also possible.

BRIEF DESCRIPTION OF DRAWINGS

In the following text, further embodiments will be described in detailwith reference to the attached drawings. These drawings are used forillustration only and do not in any way limit the scope of theinvention. In the drawings:

FIG. 1 shows a schematic internal combustion engine;

FIG. 2 shows a diagram illustrating the variation of cylinder pressureover crank angle for HCCI- and SI-mode;

FIG. 3 shows a diagram illustrating the variation of lambda in relationto engine load and engine speed in HCCI-mode;

FIG. 4 shows a diagram for a lambda control loop for combustion controlin HCCI-mode.

FIGS. 5A-B shows diagrams of an engine provided with different types ofintake air charging devices.

FIG. 1 shows a schematic illustration of an internal combustion engineaccording to the invention. The engine is provided with at least onecylinder 1 and comprises a fuel injector 2, through which fuel isinjected into a combustion chamber 3, for each cylinder. A fuelinjection control unit 4 controls fuel injection quantity per combustioncycle injected through each fuel injector. A piston 5 in the enginecylinder has a compression action that causes a mixture of air and fuelwithin the combustion chamber to be ignited during HCCI-mode. Thecylinder is provided with at least one inlet valve 6 for admitting gaswhich includes fresh air into said cylinder and at least one exhaustvalve 7 for exhausting combusted gases from said cylinder. Air issupplied through an intake conduit 9 connected to an intake manifold,while exhaust gas is exhausted through an exhaust conduit 10. DuringSI-mode, the ignition of the fuel/air mixture is ignited by a spark plug8.

The control unit receives signals from at least one sensor for measuringengine operation parameters, which sensors may include a combustionchamber pressure sensor 11, an intake manifold pressure sensor 12 and aλ-probe 13 in the exhaust conduit, as well as a temperature sensor forintake air 14, an engine load sensor 15 and an engine speed sensor 16.The control unit controls the intake and exhaust valves 6, 7 by means ofvalve actuators 17, 18. The actuators may be either electrically ormechanically operated.

According to an alternative embodiment, the λ-probe 13 can be replacedor supplemented by a NO_(x)-sensor or an ion current sensor, whichgenerates a signal indicative of the λ-value. The function of the lattersensors will be described in connection with FIG. 4 below. Similarly,the sensors for engine load and speed 15, 16 may be replaced orsupplemented by temperature sensors for engine coolant and engine oilFIG. 2 shows a diagram illustrating the variation of cylinder pressureover crank angle for HCCI- and SI-mode. As can be seen from the curvesin the diagram, the engine can be operated in homogeneous chargecompression ignition (HCCI) combustion mode and in conventional sparkignited (SI) combustion mode. The HCCI combustion has no moving flamefront, as opposed to a SI combustion that has a moving flame front. Thelack of a flame front reduces temperature and increases the heat releaserate hence increases the thermal efficiency of the combustion. This willresult in a considerably higher peak pressure after ignition (IG);typically in excess of 40 bar, as opposed to about 20 bar in SI mode.The main difference between the HCCI- and SI modes is that a part of thecombustion residuals are captured by operating the engine with anegative valve overlap. The negative valve overlap is achieved byclosing the exhaust valve, or EV, before piston TDC (EVC) and openingthe inlet valve, or IV, after piston TDC (IVO) in the gas exchange (GE)phase of the combustion, as illustrated in FIG. 2. During the air intakephase, residuals increase the temperature of the mixture so that theauto ignition temperature is reached before piston top dead center (TDC)and dilutes the mixture so that the heat release rate decreases to anacceptable level. By controlling the heat release, noise and knockingcombustion can be reduced.

A split fuel injection is used having a pilot direct fuel injection (PI)before TDC during the negative valve overlap and a main direct fuelinjection (MI) after TDC of the negative valve overlap. The relativequantities of fuel injected during the pilot and the main injections canbe varied and are calculated and controlled by a fuel injection controlunit (not shown). The fuel of the pilot injection (PI) will react withthe retained residuals, forming intermediates or combustion products.This reaction can be exothermic hence heating the residuals, resultingin earlier timing of the auto ignition temperature. The total quantityof injected fuel for the pilot and the main injection is substantiallyconstant with respect to the current engine speed requirements. Thequantity of the first injection is selected to be in the range of 10-45%of the total amount of injected fuel.

The above example describes a split fuel injection occurring before andafter top dead center of the piston stroke during the interval when bothof the exhaust and intake valves are closed. However, the invention isnot limited to this particular embodiment of split injection timing.

Due to the demand for dilution, which controls the rate of heat release,only the part load regime of the engine is used for HCCI combustionmode. The auto ignition timing for HCCI operation can be controlled bythe pilot fuel injection and/or the captured amount of residuals and/orthe absolute manifold pressure. The latter is controlled by increasingor decreasing the pressure of the intake air by means of a compressor orturbocharger.

According to one embodiment, the amount of trapped residuals duringnegative overlap should be in the range 20-60%, irrespective of how thisis achieved.

When operating the engine, it is desirable to avoid engine knocking, lowcombustion stability and a high noise level. Knocking, which is also asource of noise, is detected by measuring the peak pressure and/orpressure variations caused by a too rapid heat release during theexpansion phase. Knocking occurs when the peak pressure exceeds anexpected maximum pressure, or when a series of rapid pressure variationsoccur during the expansion phase. Low combustion stability is indicatedby high cycle to cycle variations of the pressure during combustion.Typically, an engine operated in HCCI mode may oscillate between a latephased combustion (low cylinder pressure) and a subsequent early phasedcombustion (high cylinder pressure). However, the above conditions areof less importance for the current invention.

FIG. 3 illustrates a schematic λ map to be stored in a control unit. Asseen from the figure, the air/fuel ratio λ is approximately=1.5 atidling speed under a low load. If either the load or the engine speed isincreased, while the other parameter is kept substantially constant,then the air/fuel ratio is increased to approximately λ=2. When bothengine speed and engine load are increased according to the linearfunction shown, then the air/fuel ratio is increased to approximatelyλ=2.3.

FIG. 4 shows a schematic diagram for a control strategy for managing theNO_(x) emissions by adjusting the absolute pressure in the air intakemanifold. The adjustments are made based on a comparison between themeasured, actual lambda value and a lambda value selected on the basisof a number of sensor readings. It is not possible to adjust the intakemanifold pressure from cycle to cycle. The control strategy involves anevaluation of the output signals from multiple sensors, primarily anengine speed sensor, an engine load sensor and pressure and temperaturesensors in the intake manifold.

When the engine is switched to HCCI-mode the NOx control loop S1 isinitiated by a control unit. In a first step S2, the control unit readsthe signals transmitted from a λ-sensor, an engine speed sensor, anengine load sensor, an intake manifold pressure sensor and an intake airtemperature sensor. Further sensors may include temperature sensors forengine oil and coolant.

When receiving the signals for engine speed and engine load, the controlunit will use these values to look up and select an associated value forlambda λ_(s) in a map S3 stored in the control unit. The control unitwill then check the signal from the intake manifold temperature T_(M)sensor and correct the selected λ-value S4, if the measured temperatureT_(M) deviates from a predetermined reference temperature T_(REF) forsaid stored map. In general, if the intake manifold temperature T_(M)increases the λ-value will need to be corrected upwards. Such acorrection may also be required for increasing temperatures for engineoil and coolant, which contributes to a general increase in temperatureof the engine.

The control unit will then compare the selected λ_(s) with a measuredλ-value λ_(A) for the air fuel ratio in the exhaust gas S5. Ifλ_(A)>λ_(S) then the control unit will transmit a signal to the intakeair charging unit to decrease the absolute intake pressure S6.Similarly, if λ_(A)<λ_(S) then the control unit will transmit a signalto said air charging unit to increase the absolute intake pressure S7.

Before repeating the control loop, the control unit checks that theHCCI-mode is still in operation S8. If this is not the case, then theNO_(x) control ends S9.

In the above example the λ-value is measured using a λ-sensor, such asan oxygen sensor. However, it is also possible to use sensors thatgenerate a signal indicative of the λ-value, such as a NO_(x)-sensor oran ion current sensor. Hence, for the step S5 above, either of a NO_(x)signal or ion current signal may be compared to a respective referencesignal. If either signal is lower than its reference signal, the controlunit will transmit a signal to the intake air charging unit to decreasethe absolute intake pressure S6, and vice versa. According to a furtherembodiment, the value of λ may be calculated by the ECU, using the massair flow (MAF) and the amount of injected fuel.

High λ-values combined with low fuel consumption and low NO_(x)emissions can not be achieved with atmospheric pressure in the intakemanifold during HCCI operation at high loads and/or speeds. In oneembodiment the engine is provided with an intake air charging unit,arranged to adjust the intake manifold pressure.

FIGS. 5A and 5B show different air charging arrangements for an engine Eas described in connection with FIG. 1. According to FIG. 5A, the intakeair charging unit may be a turbocharger 20 having a compressor 21 forintake air 22 and a turbine 23 driven by exhaust gases 24 from theengine E, which turbine is provided with a wastegate for controlling theintake manifold pressure. According to one embodiment, this wastegate 25may be connected to bypass the turbine 23. However, according to analternative embodiment a wastegate 26 may be connected to bypass thecompressor 21, as indicated by dotted lines in the figure. Acontrollable valve 27 may also be connected to the intake conduitbetween the compressor 21 and the engine E, to exhaust compressed air tothe atmosphere.

FIG. 5B shows an alternatively intake air charging unit in the form of asupercharger 28 for intake air 29. As in the case of the turbocharger inFIG. 5A, the supercharger is provided with a throttle or wastegate forcontrolling the intake manifold pressure. A wastegate 30 may beconnected to bypass the supercharger 28, as indicated by dotted lines inthe figure. A controllable valve 31 may also be connected to the intakeconduit between the supercharger 28 and the engine E, to exhaustcompressed air to the atmosphere.

According to one embodiment, the intake air charging unit is arranged toincrease the intake manifold pressure if the actual λ-value is less thanthe selected λ-value. Similarly, the intake air charging unit isarranged to decrease the intake manifold pressure if the measuredλ-value is greater than or equal to the selected λ-value.

The invention is not limited to the embodiments described above and maybe varied freely within the scope of the appended claims.

1. An internal combustion engine provided with at least one cylinder andcomprising: a fuel injector, through which fuel is injected into acombustion chamber, for each cylinder; a piston in the engine cylinderwhose compression action causes a mixture of air and fuel within thecombustion chamber to be ignited; at least one inlet valve for admittinggas which includes fresh air into said cylinder; at least one exhaustvalve for exhausting combusted gases; a pressure sensor for measuringintake manifold pressure and generating a pressure signal; a sensor forgenerating a signal representing an actual λ-value; sensors formeasuring at least one further engine operation parameter, and a controlunit that controls fuel injection quantity per combustion cycle injectedthrough each fuel injector, and during compression ignition mode, thecontrol unit is arranged to select a λ-value from a map stored in thecontrol unit, which value is a function of engine load and engine speed,and to compare the actual λ-value with the selected λ-value; whereby thecontrol unit is arranged to adjust the intake manifold pressure as afunction of the difference between the said λ-values in order to obtainthe selected λ-value.
 2. The internal combustion engine according toclaim 1, wherein the engine is provided with an intake air chargingunit, arranged to adjust the intake manifold pressure.
 3. The internalcombustion engine according to claim 2, wherein said λ-sensor isarranged to measure an air/fuel ratio and generate a signal representingthe actual λ-value.
 4. The internal combustion engine according to claim3, wherein the intake air charging unit is a turbocharger.
 5. Theinternal combustion engine according to claim 4, wherein theturbocharger is provided with a wastegate for controlling the intakemanifold pressure.
 6. The internal combustion engine according to claim3, wherein the intake air charging unit is a supercharger.
 7. Theinternal combustion engine according to claim 6, wherein thesupercharger is provided with a throttle for controlling the intakemanifold pressure.
 8. The internal combustion engine according to claim3, wherein the intake air charging unit is arranged to increase theintake manifold pressure if the actual λ-value is less than the selectedλ-value.
 9. The internal combustion engine according to claim 3, whereinthe intake air charging unit is arranged to decrease the intake manifoldpressure if the measured λ-value is greater than or equal to theselected λ-value.
 10. The internal combustion engine according to claim2, wherein a NO_(x)-sensor is arranged to generate a signal representingthe actual λ-value.
 11. The internal combustion engine according toclaim 2, wherein an ion current sensor is arranged to generate a signalrepresenting the actual λ-value.
 12. The internal combustion engineaccording to claim 1, wherein the engine is provided with a sensor formeasuring current engine load and a sensor for measuring engine speed.13. The internal combustion engine according to claim 11, wherein thecontrol unit is arranged to adjust the stored map of λ-values based ameasured intake air temperature.
 14. The internal combustion engineaccording to claim 1, wherein the engine is provided with a sensor formeasuring engine oil temperature and a sensor for measuring coolanttemperature.
 15. A method for operating an internal combustion engine ina compression ignition mode of operation, the engine provided with atleast one cylinder, the engine having a fuel injector, through whichfuel is injected into a combustion chamber, for each cylinder; a controlunit that controls fuel injection quantity per combustion cycle injectedthrough each fuel injector; a piston in the engine cylinder whosecompression action causes a mixture of air and fuel within thecombustion chamber to be ignited during compression ignition mode; atleast one inlet valve for admitting gas which includes fresh air intosaid cylinder; at least one exhaust valve for exhausting combustedgases; a sensor for measuring intake manifold pressure and generating apressure signal; a sensor for generating a signal representing an actualλ-value; and sensors for measuring current engine load and engine speed,the method comprising: selecting a λ-value from a map stored in thecontrol unit, which value is a function of engine load and engine speed;comparing the actual λ-value with the selected λ-value; and adjustingthe intake manifold pressure as a function of the difference between thesaid λ-values in order to obtain the selected λ-value.
 16. The methodaccording to claim 15 further comprising using a NO_(x)-sensor togenerate a signal representing the actual λ-value.
 17. The methodaccording to claim 15 further comprising increasing the intake manifoldpressure if the measured λ-value is less than the selected λ-value. 18.The method according to claim 17, further comprising decreasing theintake manifold pressure if the measured λ-value is greater than orequal to the selected λ-value.
 19. The method according to claim 18further comprising measuring intake air temperature and adjusting theλ-values in said stored map with respect to the measured temperature.20. The method according to claim 15 further comprising adjusting theintake manifold pressure using an intake air charging unit.
 21. Themethod according to claim 20 further comprising using an intake aircharging unit in the form of a turbocharger.
 22. The method according toclaim 20 further comprising controlling the intake air charging unitusing a throttle or a wastegate.
 23. The method according to claim 20further comprising using an intake air charging unit in the form of asupercharger.
 24. The method according to claim 20 further comprisingcontrolling the intake air charging unit using a throttle.